The flooding that damaged as many as 110,000 homes and more than 100,000 vehicles in Louisiana last month cost $10 billion to $15 billion, with most of the sum uninsured, according to a risk modeler.

Private residential policies don't protect against flooding in the U.S., and at least 80 percent of the damaged homes lacked coverage through a government program, according to a report Friday from Impact Forecasting, an arm of insurance broker Aon.

The disaster was the costliest of the year in the U.S., which is known for higher-than-average rates of insurance protection for more frequent perils, such as wind damage, hailstorms and fires. Most of the losses were also uncovered in Italy, where an earthquake in August killed more than 290 people, according to Aon, which didn't provide a specific estimate for costs there.

"While both these events were multi-billion dollar disasters, unfortunately, the vast majority of the losses are likely to be uninsured, further exposing the reality that certain perils remain vastly under-insured regardless of region," Steve Bowen, a director and meteorologist at Impact, said in a statement. "Indeed, as we enter the final third of 2016, roughly 75 percent of the year's disaster losses have been uninsured."

The most expensive disaster in the U.S. this year before the flooding was in April, when storms including hail and tornadoes struck Texas and other states, costing more than $3 billion, according to Aon.

Still, the S&P 500 Property & Casualty Insurance Index has advanced about 7.4 percent this year, maintaining gains since the end of July as the industry was spared most of the costs of the flooding, and a tropical storm this month did less damage than feared.

"The storm that occurred last weekend was, obviously, a lot less of an event than anticipated," Beth Bombara, the chief financial officer of Hartford Financial Services Group Inc., said in a conference Wednesday. "So, we feel really good about where we sit today and we just have to see how the remainder of the quarter turns out."